Abstract's Details

Probing Reaction Dynamics in Non-crystalline Phase by Using Ultrafast X-ray Diffraction
Abstract ID	W:LED-06
Presenter	Hyotcherl Ihee
Presentation Type	Laser Excited Dynamics
Full Author List	H. Ihee (1)
Affiliations	(1) Korea Advanced Institute of Science and Technology
Category	Instrumentation/Development
Abstract	In this talk, I will summarize the current status and limitations of time-resolved diffraction studies of solution-phase photochemistry using third-generation synchrotrons and discuss various issues related with the future use of XFEL. The importance of gas phase studies will be also briefly mentioned. The chemistry in solution phase is a very important field of research as it is a natural environment for the majority of life processes. Studying this environment with X-rays is ideal since X-rays can penetrate bulk samples and hence reveal the elementary principles driving chemical reactions. Because the atoms are so small (~10^-10 m), and they take very little time to change positions (~10^-13 s) their direct observation has eluded scientists for decades. Part of this feat has been accomplished by utilizing pulsed character of third generation synchrotrons with the pump-probe method, where a pump laser triggers a reaction and a delayed “probe” X-ray pulse captures a snapshot of the transient species. Technological innovations in synchrotron instrumentation in third generation synchrotrons and the development of novel data analysis have made it possible to track increasingly complex reactions in solution by time-resolved diffraction to a temporal and spatial resolution of 100 ps and 0.001 Å, respectively. For instance, the photodissociation reactions of various molecules such as C₂H₄I₂, HgI₂, CHI₃, I₃^-, Ru₃(CO)₁₂ in the solution phase have recently been investigated by this technique and the data analysis revealed complex reaction pathways and molecular structures of transients along the reaction pathways. In addition, we further extended this technique to study protein structural dynamics in solution. We obtained time-resolved WAXS (wide-angle x-ray diffraction) data from heme proteins such as myoglobin and hemoglobin and preliminary analysis showed that the 3D structural changes of proteins can be monitored by time-resolved x-ray diffraction even in solution. With future XFELs, the time resolution of ultrafast diffraction will be much improved from 100 ps to 100 fs, reaching the time scale of the bond-breaking and bond-making. Therefore it should be possible to track all atomic movement in real time, at least in principle. However, the success of such studies will depend not only on the experimental realization using XFELs, but also on the development of adequate theory which can explain the time-resolved diffraction signal in sub-picosecond time scale. With the current 100 ps time resolution, the analysis of the time-resolved data has been greatly simplified by the fact that the system can be approximated as quasi-equilibrium. The difficulty of studying solution phase photochemistry comes from the fact that we have to consider the contributions not only from the solute molecules but also the surrounding solvent molecules and the solute-solvent cross-term. In this regard, it would be more appropriate to conduct the first photochemistry experiment in the gas phase rather than in the solution phase since in the former only the naked solute molecules need to be considered.
Footnotes
Funding Acknowledgement	This work was supported by National Creative Research Initiatives (Center for Time-Resolved Diffraction) of MOST/KOSEF.